(1) Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, the invention relates to a low fuel consumption tire with all weather running performances in which particularly, wet skid resistance, rolling resistance, wear resistance and running performance on ice and snow road are remarkably improved.
(2) Description of Prior Art
Since a styrene-butadiene copolymer rubber is excellent in wet skid resistance on wet roads and wear resistance, this rubber has been heretofore used widely for the treads of the tires. However, when such styrene-butadiene copolymer rubber is used, the tread is difficult to be applied to low fuel consumption tires because energy loss is large, heat is likely to be generated, and energy loss in rolling is large. Further, since the rubber hardening is conspicuous at a low temperature, the skid performance on the ice and snow road is deteriorated and the application of this rubber to the tire for use in the snow and ice road is not desirable.
As the method of simultaneously improving the wet skid resistance, the heat generation, and the rolling resistance, Japanese Patent Application Laid-Open No. 51,503/1982, Japanese Patent Application Laid-Open No. 55,204/1982, and Japanese Patent Application Laid-Open No. 36,705/1982 disclose the use of styrene-butadiene rubber (SBR) in which the contents of bound styrene and vinyl bonds are controlled. This SBR exhibits an excellent wet skid resistance and low fuel consumption, but the glass transition temperature (Tg) is high and the rubber hardening is rapid and conspicuous at a low temperature. Further, the deterioration in the brittle fracture performance at a low temperature is observed, thereby resulting in the deteriorated skid performance on the snow and ice road.
On the other hand, although there have been proposed various pneumatic tires suitable for the snow and ice roads, the performances thereof are not necessarily satisfactory, and still desired to be improved. In this connection, examination has been heretofore made on the material of the tread member to enhance the tread-gripping force on the snow and ice roads. The techniques of reducing the hardness of rubber at low temperatures are well known from C. S. Wilkinson; RCT, 27 255 (1954), F. S. Conant; RCT 22 863 (1949); and so on. For instance, as known from the literatures such as W. G. Norich: The friction of Polymer on Ice (Journal of the IRI, pp 192, October, 1972), Desmond Moore; The friction of Pneumatic Tires (1975) and so on, when butadiene rubber (BR), natural rubber (NR), and/or polyisoprene rubber (IR) is used as the tread rubber, the tread-gripping force on the snow and ice road is improved. However, such method unfavorably lowers the tread-gripping force on the wet roads. In order to mitigate the reduction in the tread-gripping force, as disclosed in the literatures such as A. C. Bassi; RCT 381 965, D. Bulgin, G. D. Hubberd, M. H. Walters; Proc. 4th Rubber Tech. Conf. London 193 (1962) and so on, there has been known a method of using a large amount of styrene-butadiene copolymer rubber, butyl rubber or carbon black. However, such a method unfavorably lowers the snow and ice performances and increases the rolling resistance.
Moreover, Japanese Patent Publication No. 46,257/1983 discloses that the ice skid resistance, the wet skid resistance and the wear resistance are enchanced by using in the tires for trucks and buses a tread rubber composition in which a three kind rubber blend of NR, SBR containing not more than 20% by weight of styrene and BR is added with an extender oil of a flowing point of not more than -10.degree. C. in an amount of 20% by weight relative to all the extender oil. When such a tread rubber composition is used, the ice skid resistance, the wet skid resistance and the wear resistance of the truck and bus tires are improved to some extent, but the low fuel consumption, the wet skid resistance and the snow/ice road running performances which are aimed at in the present invention and are required mainly in the tires for passagers cars are considered to be still insufficient in light of the required levels thereof.
The present inventors et al proposed in Japanese Patent Application No. 79,406/1983 and Japanese Patent Application No. 79,407/1983 a pneumatic tire using a rubber composition containing a low temperature plasticizing ester in the tread with view to drastically enhancing the tread-gripping force on the snow and ice roads without lowering the wet skid resistance, the wear resistance, and increasing the rolling resistance.
Although the wet skid resistance and the rolling resistance are desired to be further improved in order to develop the low fuel consumption tire with the so-called all weather running performance in which the rolling resistance, the wet skid resistance, and the snow/ice road running performances are further improved, it is an actual situation that there has been available up to now no such a low fuel consumption tire with all the weather running performances.